The National Endowment for the Arts has eliminated five established grant programs, including one that provides money for museums to purchase the work of living American artists.

Museum directors and working artists say that the museum-purchase program had, in fits and starts during the past 20 years, become one of the most important mechanisms by which emerging artists made their first sales to legitimate museum collections.

The NEA, in a letter sent to dozens of museum grant applicants on Dec. 20, said elimination of the programs was the result of the complicated political accommodation that renewed the endowment's legislative mandate last fall and appropriated money for the agency for fiscal year 1991.

Andrew Oliver Jr., director of the NEA's museum program, who signed the letter, said in a telephone interview yesterday, "We realize that this will be painful."

Not only is the money gone for 1991, he said, but the NEA already has decided not to offer the category in 1992, when additional transfers to state arts councils will be required. And, said Oliver, the museum-purchase program -- founded in 1971 but suspended from 1981 to 1987 under NEA budget cuts in the Reagan administration -- may never be resumed.

"It would be wrong for me to arouse any sort of optimism at this point," he said. "Not until Congress appropriates additional sums of money for the agency as a whole are we likely to see this program come back."

Oliver's letter noted that the new legislation governing the arts endowment requires that the agency nearly double, by 1993, the proportion of its budget channeled directly to state arts councils. As a result, existing NEA grant programs are under great pressure to make cuts.

The five NEA grant categories eliminated are the museum-purchase plan, which was budgeted to distribute $698,500 this year; another museum program that supported special programs to help museums redefine their missions and artistic directions, which was to distribute $525,000 ; $305,000 to support special programs for national and regional multimedia arts organizations; $50,000 for other special projects; and $785,000 in project funds for state arts agencies -- money transferred directly to the state councils.

But, according to some artists, gallery operators, museum officials and arts observers across the country, it is elimination of the program that helps museums purchase the work of living American artists that may be the most troubling.

At the University Art Museum at California State University, Long Beach, officials said that the program constituted the principal -- and in some cases only -- way to expand collections for smaller and medium-size urban art museums and most museums that serve primarily rural areas.

"To all intents and purposes, it wipes out our acquisition program," said Constance Glenn, the museum's director. Cal State Long Beach, she said, received purchase grants repeatedly the past few years and had hoped for $20,000 in NEA funds for 1991.

"The art world is very unstable today," Glenn said. "People aren't making large gifts right now or making impromptu gifts."

In Chicago, Denise Miller-Clark, director of the Museum of Contemporary Photography at Columbia College, said that elimination of an $8,325 grant that the museum had sought to buy work by three young, but established, photographers would probably wipe out her museum's ability to buy any work this year.

"We have to make a very small budget go far and we have utilized the NEA grant program to help us make significant purchases," she said.

"This is kind of the risk money for a museum," said Joy Silverman, interim director of the National Campaign for Freedom of Expression, an artist activist group. "What it provides is a kitty of money that's usually matched by {the fund-raising of} museum board members who are interested in taking chances on new work and new ideas.

"This isn't like purchasing a Rembrandt, where history has already proven that this artist is accomplished. If the national endowment is not encouraging this kind of activity at all, it's going to be harder everywhere."